Direct microscopic observation of arterial vessels just proximal to the capillary bed will be made during and following perfusion of these vessels with barbiturates commonly used by anesthesiologists and those occasionally used by drug addicts. The exact mechanism of the severe damage to the circulation resulting from inadvertant intra-arterial injections of these materials is unknown, and clinical attempts to re-institute blood flow have not been uniformly successful. In pilot experiments, direct inspection of the microcirculation has revealed occlusion of small arterial vessels with platelets. Following the determination of the cause of impairment to the circulation, attempts will be made to re-establish blood flow with various agents.